Oscillating devices are generally applied to electronics that require a stable output frequency, such as mobile phones. Such oscillating device mostly incorporates an AT-cut crystal oscillator in a frequency band of 10 MHz as the oscillation source to constitute an oscillation circuit. Because the output frequency of the AT-cut crystal oscillator varies with the environmental temperature, it is necessary in practice to design a temperature compensation circuit for eliminating the frequency variation of the AT-cut crystal oscillator.
FIG. 1 illustrates the relation between the frequency deviation of an AT-cut crystal oscillator and the environmental temperature. As shown in FIG. 1, the relation between the output frequency of the AT-cut crystal oscillator and the environmental temperature appears to be a cubic curve such as ƒ=αT3+βT2+γT+δ. The cubic-curve can be divided into three temperature regions, i.e., low-temperature middle temperature and high-temperature. The cubic curve includes a first linear region of positive slope and a first nonlinear region with an inflection point in the low-temperature region (from −35° C. to +10° C.), a second linear region of negative slope in the middle-temperature region (from +10° C. to +50° C.), and a third linear region of positive slope and a second nonlinear region with an inflection point in the high-temperature region (from +50° C. to +90° C.).
FIG. 2 shows a circuit of an oscillating device 10 according to the prior art. As shown in FIG. 2, the oscillating device 10 includes a temperature detection circuit 12, an oscillation circuit 20 and a temperature compensation circuit 40. The oscillation circuit 20 includes an AT-cut crystal oscillator 22, a feedback resistor 24 and an inverter 26 connected to the AT-cut crystal oscillator 22 in parallel. The output port 28 of the oscillating device 10 is pulled out from the output terminal of the inverter 26. The oscillation circuit 20 further includes two DC cut-off capacitors 32, 34 and two variable capacitors 36, 38 connected two terminals of the AT-cut crystal oscillator 22, respectively. The temperature detection circuit 12 uses a thermistor to detect the environmental temperature of the AT-cut crystal oscillator 22, and the temperature compensation circuit 40 maintains the output frequency of the oscillation circuit 20 to a predefined value according to a temperature detection signal from the temperature detection circuit 12.
The temperature compensation circuit 40 includes a memory circuit 42 and a digital/analog conversion circuit 44. The memory circuit 42 is usually composed of the nonvolatile memory to store data required for the temperature compensation, i.e., parameters for describing the cubic curve. According to the compensation data recorded in the memory circuit 42 and the temperature detection signal from the temperature detection circuit 12, the digital/analog conversion circuit 44 outputs a control voltage to the positive electrodes of the variable capacitors 36, 38, respectively, to adjust its oscillation capacitance. Consequently, the oscillation frequency of the oscillation circuit 20 can be controlled and the frequency deviation of the oscillating device 10 can be maintained within an allowable range of the product specification.
Since the AT-cut crystal oscillator 22 is cut mechanically (by laser), the thickness and cutting angle of each AT-cut crystal oscillator 22 are not identical completely each time, which makes its temperature-frequency characteristic different each time. Similarly, the electronic devices of the oscillating device 10 also have characteristic difference originated from the process drifting. In short, the oscillating device 10 is different each time due to the difference of the manufacture procedure, and the temperature-frequency characteristic of each oscillating device 10 is therefore different each time. Therefore, the temperature-frequency characteristic of each oscillating device 10 must be measured in operation temperature regions (high, middle and low temperature regions), and must write the temperature compensation data into the memory circuit 42. However, testing the temperature-frequency characteristic for each oscillating device 10 individually is a very time-consuming work, which results in a dramatic increase in the total testing cost of the oscillating device 10.